1. Field of the Invention
This invention relates generally to a system and method for using regenerative braking power to start a fuel cell stack and, more particularly, to a system and method for using regenerative braking power during a start/stop operation of a fuel cell hybrid vehicle to restart a fuel cell stack.
2. Discussion of the Related Art
Most fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental high voltage power source in addition to the fuel cell stack, such as a DC battery or an ultra-capacitor. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to an electric traction motor that drives the vehicle and other vehicle systems through a high voltage bus for vehicle operation. The battery provides the supplemental power to the voltage bus during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand.
Regenerative braking can cause the electric traction motor in the electric hybrid vehicle to operate as a generator to convert rotational energy from the vehicle wheels to electrical power that can be used to charge the battery in a manner that is well understood by those skilled in the art. However, in known electrical vehicles, the collection of regenerative braking power is prevented during fuel cell system starts.
Some hybrid vehicles employ a start/stop mode where only battery power is used to provide the power demands for the vehicle, where the fuel cell stack is disconnected from the high voltage bus. For example, during idle conditions when low power is required, the fuel cell stack may be disconnected from the high voltage bus and the cathode compressor may be turned off, or at least operated at a low speed. Further, during long travel downhill where significant regenerative braking may be provided, fuel cell stack power may not be required and the fuel cell stack can be disconnected from the high voltage bus for at least a certain amount of time.
At some point it becomes necessary to reconnect the fuel cell stack to the high voltage bus because more power is necessary, the internal temperature of the stack has fallen to some minimum temperature, or some other system requirement has been met. In known start/stop modes, battery power is used to spin up the cathode compressor and provide other electrical needs to begin stack operation. If the vehicle is currently in a regenerative braking mode, it may be possible to use the regenerative power to provide those electrical requirements and reduce the load on the battery.